The present disclosure relates in general to three-dimensional (3D) surface contouring techniques, and more particularly to a device and method for using a spatial light modulator as a dynamic diffraction grating to reflect structured light in one of many types of patterns onto a surface of an object to ultimately determine through triangulation the 3D contour of the object's surface using the interference of two beams or spots of light.
In the field of three-dimensional surface contouring for accurately and rapidly determining the 3D coordinates of an object, there are many known techniques available, some of which involve the use of projecting various structured light patterns onto the object. The structured light pattern is typically formed in fringes (i.e., alternating bright and dark or different colored “stripes” or regions) on a surface of the object. In some cases, a spatial light modulator in the form of a diffraction grating of either a transmissive or reflective type is used to form grating patterns and to vary the phase of these patterns. The resulting fringe patterns on the surface of the object are then viewed by a camera device such as a charge coupled device (CCD), and processed by a computer or processor using various known triangulation techniques to ultimately determine the 3D surface contour of the object.
However, drawbacks with this type of approach include the fact that the diffraction grating is of a “static” type which must be moved by some type of manual means to effectuate a shift in the phase of the grating patterns. This results in a relatively slow phase shifting speed, which leads to less than optimum performance of the overall system. Also, such a system may require multiple separate diffraction gratings, each having a different grating period, to create a fringe pattern having the required spacing between the fringe lines (also known as pitch of the fringe lines). Besides the multiple gratings, it may also be necessary to provide associated translation stages and optical component feedback mechanisms, both of which are generally relatively expensive. Such a system may also require a relatively large amount of processor capability to process the camera captured images.
Other known prior art 3D object surface contouring systems are based on the direct projection of laser light, the projected image being essentially a replica of a pattern formed in a spatial light modulator such as, for example, in a digital micromirror device.
It is desirable to create very pure sinusoidal patterns having an infinite depth of field. A way to do this is to use a reflective or transmissive device as a dynamic diffraction grating device in a relatively highly accurate and less expensive 3D object surface contouring measurement system to form various types of structured light patterns by reflection of light off of the grating which then provides the reflected light through a pinhole plate to create by filtering two focused spots of light corresponding to the +1 and −1 order modes, and then allowing the light from the two spots of light to interfere at the surface of an object. The interference creates periodic sine waves that vary in intensity, thereby representing fringe patterns whose images may then be captured by a camera device and processed using known triangulation techniques to determine the 3D surface contour of the object. The reflective dynamic diffraction grating may comprise a digital micromirror device (DMD) comprised of a two-dimensional array of a plurality of movable reflective light switches or mirrors formed using microelectromechanical systems (MEMS) technology. The dynamic diffraction grating may be referred to in general as a spatial light modulator (SLM) of which the grating may be a particular type of SLM.